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Abstract:

A method is provided for controlling a wind farm connected to a grid. The
wind farm includes a wind farm emergency generator and at least a first
wind turbine. The first wind turbine includes a wind turbine transformer,
a wind turbine converter and auxiliary equipment. The method includes
detecting a lost grid connection, providing active power to the auxiliary
equipment via the wind farm emergency generator, providing reactive power
via the wind turbine converter.

Claims:

1. A method for controlling a wind farm connected to a grid, wherein the
wind farm comprises a wind farm emergency generator, and at least a first
wind turbine, wherein the first wind turbine comprises a wind turbine
transformer, a wind turbine converter and auxiliary equipment, wherein
the method comprises: detecting a lost grid connection, providing active
power to the auxiliary equipment via the wind farm emergency generator,
and providing reactive power via the wind turbine converter.

2. The method for controlling a wind farm as set forth in claim 1,
wherein the wind farm further comprises a wind turbine switch for
connecting a second wind turbine, wherein the method further comprises:
closing the wind turbine switch for connecting the second wind turbine.

3. The method for controlling a wind farm as set forth in claim 2,
wherein providing reactive power via the wind turbine converter comprises
providing reactive power for energizing a wind turbine transformer of the
second wind turbine.

4. The method for controlling a wind farm as set forth in claim 1,
wherein providing active power to the auxiliary equipment via the wind
farm emergency generator comprises ramping up slowly a wind farm
emergency generator voltage from zero to nominal.

5. The method for controlling a wind farm as set forth in claim 1,
wherein the wind farm comprises a shunt reactor, in particular a variable
shunt reactor, wherein the method further comprises, compensating
reactive power via the shunt reactor.

6. The method for controlling a wind farm as set forth in claim 1,
wherein the wind farm comprises a static synchronous compensator
(STATCOM), wherein the method further comprises compensating reactive
power via the static synchronous compensator.

7. A wind farm controller adapted to perform the method according to
claim 1.

8. A wind farm, comprising: a wind farm controller according to claim 7,
a wind farm emergency generator, and a first wind turbine, wherein the
first wind turbine comprises a wind turbine transformer, and a wind
turbine converter.

9. The wind farm according to preceding claim 8, wherein the wind farm
further comprises a second wind turbine, and wherein the first wind
turbine comprises a wind turbine switch for connecting the second wind
turbine.

10. The wind farm according to preceding claim 8, wherein the wind farm
further comprises a shunt reactor, in particular a variable shunt
reactor.

11. The wind farm according to preceding claim 8, wherein the wind farm
further comprises a static synchronous compensator (STATCOM).

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of European Patent Office
application No. 12161408.5 EP filed Mar. 27, 2012. All of the
applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

[0002] The embodiments illustrated herein relate to the field of methods
for controlling a wind farm. Moreover, the embodiments relate to a wind
farm controller and a wind farm. Furthermore, the embodiments also relate
to a computer-readable medium and a program element.

BACKGROUND OF INVENTION

[0003] Wind farms typically comprise several wind turbines connected to a
grid, which on the one hand transports the produced energy to the
consumers and on the other hand provides frequency, voltage and reactive
power support. Generally, each wind turbine comprises a wind turbine
transformer, a wind turbine converter, a wind turbine generator and
auxiliary equipment, e.g. measuring equipment, strobe lights, yaw motors
etc.

[0004] In case of a lost grid connection the auxiliary equipment has to be
provided with power. It may, for example, be important that yaw motors
can operate when the grid connection is lost. The yaw motors may, for
example, rotate the nacelle of the wind turbine into a position, where
the mechanical stresses are minimized. Accordingly, a central wind farm
emergency generator is provided, which provides the required active
power. However, it is necessary to energize the wind turbine transformers
first. Hence, the wind farm emergency generator has to be capable of
delivering a high amount of reactive power corresponding to the high
inrush current to all of the transformers, too.

[0005] So far this problem has been solved by using a relative large
diesel generator, which is adapted to deliver the inrush current to the
all the transformers. However, in particular in case of offshore wind
farms it may be difficult and costly to provide a platform for such a
huge diesel generator.

[0006] Hence there may be a need for a method for controlling a wind farm,
a wind farm controller, a wind farm, a computer-readable medium and a
program element allowing for smaller wind farm emergency generators.

SUMMARY OF INVENTION

[0007] This need may be met by the subject matter according to the
independent claims Example embodiments are described by the dependent
claims.

[0008] According to a first aspect, there is provided a method for
controlling a wind farm connected to a grid, wherein the wind farm
comprises a wind farm emergency generator and at least a first wind
turbine, wherein the first wind turbine comprises a wind turbine
transformer, a wind turbine converter and auxiliary equipment, wherein
the method comprises detecting a lost grid connection, providing active
power to the auxiliary equipment via the wind farm emergency generator,
providing reactive power via the wind turbine converter. For example, the
wind turbine converter may be a frequency converter. In particular, the
wind turbine converter may be a double fed and/or a full scale system.

[0009] All the elements required for performing said method are typically
available in current wind farms. Hence, the method may be easily
performed with existing wind farms.

[0010] According to a first exemplary embodiment of the method the wind
farm further comprises a wind turbine switch for connecting a second wind
turbine and the method further comprises closing the wind turbine switch
for connecting the second wind turbine. The wind turbine switch may for
example be located in the first wind turbine or in the second wind
turbine.

[0011] Connecting the wind turbines one after the other may reduce
reactive power demand peaks. Alternatively or in addition radials of wind
turbines may be connected one after the other.

[0012] According to a further exemplary embodiment of the method providing
reactive power via the wind turbine converter comprises providing
reactive power for energizing a wind turbine transformer of the second
wind turbine.

[0013] This embodiment may further reduce the reactive power that an
emergency generator has to provide in case of a lost connection to the
grid.

[0014] According to another exemplary embodiment of the method providing
active power to the auxiliary equipment via the wind farm emergency
generator comprises ramping up slowly a wind farm emergency generator
voltage from zero to nominal.

[0015] This embodiment may enhance voltage stability.

[0016] According to a still further exemplary embodiment of the method the
wind farm comprises a shunt reactor, in particular a variable shunt
reactor and the method comprises compensating reactive power via the
shunt reactor.

[0017] Hence, the reactive power loads on the wind farm emergency
generator may be further reduced. A variable shunt reactor may
continuously compensate reactive power as the load varies and thereby may
secure voltage stability. A variable shunt reactor may reduce voltage
jumps resulting from switching in and out traditional fixed shunt
reactors.

[0018] According to yet another exemplary embodiment of the method the
wind farm comprises a static synchronous compensator and the method
further comprises compensating reactive power via the static synchronous
compensator.

[0019] According to a second aspect, there is provided a wind farm
controller adapted to perform the method according to any one of the
embodiments as have been described hereinbefore.

[0020] According to a third aspect, there is provided a wind farm
comprising a wind farm controller as has been introduced hereinbefore, a
wind farm emergency generator and a first wind turbine, wherein the first
wind turbine comprises a wind turbine transformer and a wind turbine
converter.

[0021] According to a first embodiment the wind farm further comprises a
second wind turbine and the first wind turbine comprises a wind turbine
switch for connecting the second wind turbine.

[0022] According to a further embodiment the wind farm further comprises a
shunt reactor, in particular a variable shunt reactor.

[0023] According to another embodiment the wind farm further comprises a
static synchronous compensator.

[0024] According to a forth aspect, there is provided a computer-readable
medium on which there is stored a computer program for processing a
physical object, the computer program, when being executed by a data
processor, is adapted for controlling and/or for carrying out the method
as set forth above.

[0025] The computer-readable medium may be readably by a computer or a
processor. The computer-readable medium may be, for example but not
limited to, an electric, magnetic, optical, infrared or semiconductor
system, device or transmission medium. The computer-readable medium may
include at least one of the following media: a computer-distributable
medium, a program storage medium, a record medium a computer-readable
memory, a random access memory, an erasable programmable read-only
memory, a computer-readable software distribution package, a
computer-readable signal, a computer-readable telecommunications signal,
computer-readable printed matter, and a computer-readable compressed
software package.

[0026] According to a fifth aspect, there is provided a program element
for processing a physical object, the program element, when being
executed by a data processor, is adapted for controlling and/or carrying
out the method as has been described above.

[0027] The program element may be implemented as computer readable
instruction code in any suitable programming language, such as, for
example, JAVA, C++, and may be stored on a computer-readable medium
(removable disk, volatile or non-volatile memory, embedded
memory/processor, etc.). The instruction code is operable to program a
computer or any other programmable device to carry out the intended
functions. The program element may be available from a network, such as
the World Wide Web, from which it may be downloaded.

[0028] Embodiments of the present technique may be realized by means of a
computer program respectively software. However, the embodiments may also
be realized by means of one or more specific electronic circuits
respectively hardware. Furthermore, the embodiments may also be realized
in a hybrid form, i.e. in a combination of software modules and hardware
modules.

[0029] It has to be noted that embodiments have been described with
reference to different subject matters. In particular, some embodiments
have been described with reference to method type claims whereas other
embodiments have been described with reference to apparatus type claims.
However, a person skilled in the art will gather from the above and the
following description that, unless other notified, in addition to any
combination of features belonging to one type of subject matter also any
combination between features relating to different subject matters, in
particular between features of the method type claims and features of the
apparatus type claims is considered as to be disclosed with this
document.

[0030] The aspects defined above and further aspects are apparent from the
examples of embodiment to be described hereinafter and are explained with
reference to the examples of embodiment. The invention will be described
in more detail hereinafter with reference to examples of embodiment but
to which the invention is not limited.

[0036] The wind turbines 3, 4, 5, 6 may furthermore comprise wind turbine
switches 37, 38, 39, 40, which may be used to establish a connection to
further wind turbines via cables 41, 42, 43, 44 leading to the next wind
turbine on the respective radial.

[0037] Power has to be provided to the auxiliary equipment 25, 26, 27, 28
of the wind turbines 3, 4, 5, 6 even in case of a lost grid connection.
Therefore the wind farm emergency generator 7 is provided. The wind farm
emergency generator 7 is a synchronous generator, which in the case of an
offshore wind farm may be placed on an offshore platform. Synchronous
generators may provide active power with a constant frequency. In case of
a lost grid connection the wind farm emergency generator 7 will supply
the auxiliary equipment through the cables in the collector grid and the
wind turbine transformers 13, 14, 15, 16.

[0038] The active power a wind farm emergency generator may provide
typically not only depends on the rated total power of the wind farm
emergency generator but on the generated active power, too. FIG. 2 shows
a typically power capability curve of a wind farm emergency generator,
wherein the total power has been normalized to 1.

[0039] The x-axis shows the reactive power R. Negative values correspond
to reactive power import, i.e. leading power factor, and positive values
to exported reactive power, i.e. a lagging power factor. The active power
A is shown on the y-axis.

[0040] A first, second, third, forth and fifth operating region 45, 46,
47, 48 and 49, respectively, may be distinguished. The first operating
region should be avoided as voltage instabilities.

[0041] Operating the wind farm generator in the fifth operating region may
result in overheating of the rotor of the wind farm emergency generator.
Generally, the wind turbine emergency generator should be operated in the
third operating region, i.e. the acceptable steady state operating
region.

[0042] However, a wind farm emergency generator may consume/provide more
reactive power for a transient event for some seconds. For example, for
short time periods it may be operated in the second operating range 46 or
in the forth operating range 48. As a rule of thumb, a wind farm
emergency generator may carry up to 10 percent of its rated reactive
power capability in lead power factor loads without being damaged or
losing control of the output voltage. However, problems may arise if many
such transient events happen fast after each other. This may result in an
overheated generator. The most common sources of leading power factor are
lightly loaded universal power system (UPS) with input filters and power
factor correction devices for motors. Loading the wind farm emergency
generator with lagging power factor loads prior to the leading power
factor loads may improve stability.

[0043] Even in case of a lost grid connection the wind turbine
transformers have to be energized. This may result in a high reactive
inrush current, which may be several times higher than nominal current,
due to the non-linear magnetic characteristic of wind turbine
transformers.

[0044] According to the exemplary embodiment depicted in FIG. 1, in case
of a lost connection to the grid 2, the wind farm emergency generator 7
will first be connected to wind turbine 3 and its wind turbine
transformer 13 via wind turbine switch 8. The wind turbine emergency
generator 7 may cope with the reactive power demand of the wind turbine
transformer 13. Thus, the wind farm emergency generator 7 may supply the
auxiliary equipment 25 of the wind turbine 3 via the wind turbine
transformer 13.

[0045] The wind turbine converter 17 may then provide additional reactive
power. Hence, further wind turbines, in particular wind turbines 9, 10
and 11, may be connected to the grid 2 via wind turbine switches 9, 10,
11 without overloading the wind farm emergency generator 7 with reactive
power demands. The wind turbine converters 18, 19, 20 may then provide
further reactive power such that wind turbine switches 37, 38, 39, 40 may
be closed to connect cables 41, 42, 43, 44 leading to the next wind
turbine on the respective radial.

[0046] In order to recapitulate the above-described embodiments one can
state that a smaller wind farm emergency generator may be sufficient.
This may be in particular desriable in case of offshore wind farms where
limited place is available on the platforms.

[0047] While specific embodiments have been described in detail, those
with ordinary skill in the art will appreciate that various modifications
and alternative to those details could be developed in light of the
overall teachings of the disclosure. For example, elements described in
association with different embodiments may be combined. Accordingly, the
particular arrangements disclosed are meant to be illustrative only and
should not be construed as limiting the scope of the claims or
disclosure, which are to be given the full breadth of the appended
claims, and any and all equivalents thereof. It should be noted that the
term "comprising" does not exclude other elements or steps and the use of
articles "a" or "an" does not exclude a plurality.